On the left is the an image of hot (ionized)
hydrogen gas in the center of NGC 253 made by
astronomers Alan Watson (Instituto de Astronomía,
Universidad Nacional Autónoma de México),
Jay Gallagher (U. Wisconsin, Madison), John Trauger
(Jet Propulsion Laboratory) and collaborators.
The inset to the right is displayed like a photographic
negative in order to highlight 4 of the Super
Star Clusters the astronomers discovered. (These
are circled in pink and the image links to a larger
display of the hot gas image.) Their story
of this discovery follows.

The Story Behind
HST Observations of NGC 253

Our observations of the galaxy NGC 253 were
one step in a long process involving many astronomers
around the world and many telescopes, with the Hubble
Space Telescope (HST) playing a unique role since
its launch in 1990 and repair in 1993. I hope here
to briefly explain why we performed these observations
and what we learned from them.

- Alan Watson

Observations from the Ground

The motivation for our observations begins with
a 1985 paper by Chip Arp and Allan Sandage, two of the most influential
astronomers of their generation. (Chip Arp pioneered
the study of peculiar galaxies with his "Atlas
of Peculiar Galaxies". Allan Sandage has worked
for decades to determine some of the fundamental
parameters of the universe in which we live, such
as its age, rate of expansion, and whether it will
live forever).

Their paper presents a detailed study of two bright
knots of emission coincident with the dwarf galaxy
NGC 1569. (Galaxies are often referred to by their
number in the "New General Catalogue"
or "NGC" of 1888.) The two knots are unresolved
in images taken with telescopes on the Earth and
appear no different from single stars in our own
Galaxy. The question, then, was whether the knots
were simply foreground stars in our own Galaxy seen,
by chance, in front of the dwarf galaxy or perhaps
something more exciting. By analyzing the spectra
of the knots (their brightness at many different
wavelengths) Arp and Sandage were able to show that
the knots were probably not foreground stars. Instead,
they suggested that the knots were massive, compact
clusters of young stars in the dwarf galaxy itself.

Observations with the Impaired HST

The next significant developments came with the
launch of the HST in 1990. Even with its initially-impaired
optical performance, the HST gave significantly
better images than could be obtained with ground-based
telescopes. O'Connell,
Gallagher, & Hunter used the HST to obtain
images of Arp and Sandage's knots in NGC 1569. Their
images showed that these objects were resolved -
they appeared larger than the images of stars -
and so could not be foreground stars in our own
Galaxy. They had to be stellar clusters in the dwarf
galaxy itself; Arp and Sandage's suggestion had
been proven correct.

Super Star Clusters

The properties of these clusters are astounding.
They contain hundreds of luminous stars and have
masses of about half a million times the mass of
the Sun. However, all of these stars are packed
into a size of only about 5 light years, which is
about the distance from the Sun to the its nearest
neighboring star. These clusters were much brighter
and more massive than other known star clusters,
and to signify this distinction they were named
"super star clusters."

Two questions immediately attracted astronomers
to super star clusters. The first was how such clusters
could form, that is, how molecular gas, the raw
material for making clusters of stars, could be
compressed in such enormous quantities into such
a small region. This question remains a mystery.
The second was whether these clusters might be related
to the globular cluster we see in our own Galaxy
and in others. Globular clusters contain very old
stars and were probably formed when our Galaxy was
very young; by studying them we hope to learn something
about the origins of our Galaxy. When they were
young, globular clusters may well have been similar
to the super star clusters in NGC 1569. The super
star clusters may tell us something about the youth
and formation of globular clusters, and this in
turn might tell us something about the formation
of our Galaxy.

Dwarf galaxies such as NGC 1569, where super star
clusters were first discovered, are normally quite
faint and so it is relatively easy to find bright
super clusters in them. The impaired HST gave Jon
Holtzman and his colleagues the ability to discover similar clusters in the more
difficult environments of the interacting galaxy
NGC 1275. After this, many more super star clusters
were found in similar dwarf and interacting galaxies.
These observations demonstrated that super stars
clusters are not limited just to dwarf galaxies.
Astronomers started to wonder just how common they
might be.

Observations with the Repaired HST

In 1993, the Space Shuttle visited the HST and
astronauts performed space walks to install new
instruments. These new instruments corrected the
optical problems that had degraded images and allowed
the HST to perform as we had always hoped it would.

One of the questions we decided to address with
the repaired HST was just how common super star
clusters are. The impaired HST had shown that they
existed in dwarf galaxies and interacting galaxies,
but the improved performance of the repaired HST
would be required to investigate a yet more challenging
environment: the centers of starburst spiral galaxies.
These galaxies are differ from normal spiral galaxies
(like our Galaxy and the nearby Andromeda galaxy)
in that they are forming stars much more rapidly.
The brightness and richness of features in the their
central regions makes even objects as bright as
super star clusters difficult to see.

We chose to observe NGC 253, which at a distance
of 10 million light years is the nearest starburst
spiral galaxy. We took images of the galaxy only
a few months after the HST was repaired. To our
delight, the images showed
four super stars clusters. Star clusters had
now been found in all three locations where intense
star formation is taking place: dwarf galaxies,
interacting galaxies, and starburst spiral galaxies.
We concluded that the important factor was the presence
of intense star formation and that the type of galaxy
was irrelevant. That is, we expect that the formation
of super star clusters is "robust", by
which we mean that it does not depend on a special
property of the galaxy, and so we expect that they
will be formed whenever a galaxy is forming stars
at a sufficiently high rate.

Our conclusion has important implications for
the two questions mentioned earlier. The robustness
of the formation of super star clusters suggests
that the mechanism by which they form is similarly
robust; this will help constrain which theories
can and cannot explain these clusters. Secondly,
it suggests that the mechanism by which they are
currently forming may also have operated in the
past, when our Galaxy was forming globular clusters.
This strengthens the ties between super star clusters
and globular clusters and is one step further along
the road to understanding the origin of our own
Galaxy.

A Broader View

Our observations illustrate two important themes
in modern astronomy. One is that observations and
theories rarely exist in isolation, but more frequently
are steps in a long scientific process leading,
we hope, towards a better understanding of our Universe.
The other is that observations of phenomena in the
current-day Universe can often give us insight into
the early Universe in which our Galaxy was born.